1. Field of the Invention
The present invention relates to a method of fabricating a suspended structure, and more particularly, to a method of using a hardened patterned photoresist layer to be a sacrificial layer, and adjusting the sidewall shape of the sacrificial layer for fabricating a suspended structure.
2. Description of the Prior Art
Micro-electromechanical systems (MEMS) are modern technology, which coordinate electrical circuits and mechanics. MEMS have been applied to fabricate devices with both electrical circuits and mechanical elements, such as pressure sensors, accelerators, and micro-microphones. The suspended structure is applied in many MEMS devices such as a MEMS switch, an inkjet nozzle, and micro-microphones, etc., but the method of fabricating a suspended structure according to the prior art still has many limitations.
Please refer to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 show the method of fabricating a suspended structure 18 by using a wet etching process according to the prior art. As shown in FIG. 1, a substrate 10 is first provided, and the upper surface of the substrate 10 includes a patterned structure layer 14 thereon. As shown in FIG. 2, a patterned photoresist layer (not shown in the figure) is formed on the substrate 10 and the structure layer 14, and a wet etching process is substantially performed. During the wet etching process, a part of the substrate 10 is etched through the opening of the patterned photoresist layer by an etching solution, and a suspended structure 18 is therefore formed. After that, the patterned photoresist layer is removed.
In general, the material of the substrate 10 is silicon, and the wet etching process uses a potassium hydroxide (KOH) solution as an etching solution to remove the substrate 10 so as to form the suspended structure. Because the prior art method utilizes the wet etching process to form a hole under the suspended structure 18, the material of the structure layer 14 must be limited to particular metals that can sustain the etching solution. Otherwise, other material layers, such as a patterned photoresist layer, must be formed on the structure layer 14 to protect the structure layer 14. Furthermore, the prior art method cannot produce various suspended structures 18 having different inclined angles, so it cannot satisfy some required designs of the products.
On the other hand, the etching solution of the wet etching process often damages the structure layer 14, or results in ion pollution in the components. For example, the strength of the suspended structure 18 may be decreased by the surface tension of the KOH solution, so the suspended structure 18 collapses easily and sticks to the substrate 10. The substrate 10 and the structure layer 14 may be polluted by potassium ions (K+) in the KOH solution, which decreases the whole yield of the fabricating process.